The present invention relates to a method for hermetically sealing a retort container filled with various substances, in particular foods to be packaged.
Conventional sealing methods in which heat-sealing method is utilized include, for instance, one-stage sealing methods and two-stage sealing methods. The one-stage sealing method includes, for instance, a method comprising the steps of covering the opening portion of a container with a plastic film or the like and then subjecting the flanged portion of the opening to formation of ring-like line-seal to thus seal the container; a method comprising sealing whole the flanged portion of a container; and a knurled sealing method comprising adhering the whole of the flanged portion of a container in spot-like patterns.
On the other hand, techniques concerning the two-stage sealing method include, for instance, a two-stage-sealing technique which comprises contact-heating the upper side of a flange at the upper end of a container to perform the first sealing and then forming V-grooves or the like on the flange portion including the first seal to perform the second sealing as disclosed in Japanese Utility Model Unexamined Publication (hereunder referred to as "J.U.M. KOKAI") No. Sho 62-31061 which relates to the shapes of package-seal of a container for accomodating egg cooked in the form of bean curd, or the like; a two-stage sealing technique comprising the steps of heating the whole surface of a flanged portion under pressure to perform temporary sealing, then heating under pressure in the form of a ring to form a ring-like seal as disclosed in Japanese Patent Unexamined Publication (hereunder referred to as "J.P. KOKAI") No. Sho 62-193907 which relates to a method for forming a seal for a container capable of easily being opened and an apparatus for practicing the method); a method comprising performing the first sealing to form non-welded portions so that the non-welded portions or leakage paths form patterns which extend towards the outer side of the periphery of a tray, the patterns being separated from and independent of the first welded portion, and then performing the second seal by welding the outer side of the periphery so that a non-welded portion is formed as disclosed in J.P. KOKAI No. Sho 52-128790 which relates to a method for packaging a tray; and a method comprising the steps of forming the first seal so that gaps for evacuating air remaining in a package and then performing the second seal for complete sealing as disclosed in Japanese Patent Publication for Opposition Purpose (hereunder referred to as "J.P. KOKOKU") No. Sho 53-43353 which relates to a method for sealing a heat-sealable aluminum container.
It is a matter of course that the foregoing sealing is performed after contents are introduced into a container, but it is often observed that the contents are scattered during the filling operation and the scattered contents adhere to portions to be sealed, for instance, the flange portion of the container. Moreover, if a container is filled with contents up to the brim, a substantial amount of the contents often remains at the flange portion of the container. If the flange portion of the container in such a condition is sealed with a sealing material according to the foregoing sealing method, the following problems arise:
(i) If the method for forming a ring-like line-seal at the flange portion of a container is adopted, the contents of the container remain in the external and/or internal areas of the sealed part or the sealing material causes curling near the outer side of the seal, which impairs the appearance of the resulting goods.
(ii) If a method for sealing the whole of the flange of a container is adopted, it is necessary that the sealing must be performed under relatively severe conditions to ensure a sufficient sealing strength (for instance, sufficient resistance to retort). In this connection, the severe conditions mean conditions at high temperatures and/or under high pressures. Therefore, the contents remaining at the sealed portion cause foaming, this spoiling the beauty of the flange portion and the foaming results in insufficient sealing.
(iii) If the knurled sealing method for adhering the entire portion of the flange of a container in spots is adopted, it is necessary that the sealing must be performed under substantially severe conditions to ensure a sufficient sealing strength (for instance, sufficient resistance to retort). Therefore, contents remaining between the spots cause foaming which becomes a cause of insufficient sealing. For this reason, the exterior and the interior of the container being in communication with each other, and correspondingly the contents become putrefied.
(iv) If a method which comprises first performing entire sealing and then ring-sealing is adopted, contents remaning in the sealed portion cause foaming which impairs the appearance of the resulting good.